Planar piezoelectric actuator providing a large shear movement

ABSTRACT

An actuator providing a large shear movement in a chosen direction. The angle of inclination of the fibres relative to the chosen direction is larger than 2° and smaller than 40°; the spaces between the piezoelectric fibres of the active layer are filled with an incompressible elastic material; the active layer comprises at least two dimensionally stable elongate elements parallel to the chosen direction; the ends of each fibre are adhesively bonded to said dimensionally stable elements using a rigid adhesive; and said dimensionally stable elements are adhesively bonded, by a rigid adhesive, to said electrode-bearing layers.

The invention relates to an active actuator able to provide a planar shear movement. It relates more particularly to actuators using piezoelectric fibres.

Flat actuators, formed from layers of piezoelectric fibres parallel to one direction and sandwiched between layers carrying electrodes oriented perpendicularly to these fibres in order to activate them, have already been developed in order to slide two structure elements with respect to each other. The applications sought in aeronautics are for example the controlled deformation of an aerodynamic element in order to adapt it according to its variations in incidence.

In this type of application, the performance of the actuator in the shear direction is expressed by the value of the corresponding angular distortion.

The applicant has already proposed, in the document FR 2 893 783, a rectangular flat actuator producing a shear in the direction of its long edge, consisting of an active layer of piezoelectric fibres sandwiched between two layers carrying parallel electrodes, themselves each covered with a layer of fabric with rigid warp and weft oriented in two directions forming a mesh. The weft and warp define a mesh of juxtaposed deformable parallelograms. The piezoelectric fibres form an angle of 45° with the long edge and 90° with the electrodes. The various layers are adhesively bonded to each other so as best to transmit the shear movement of the planar layer of piezoelectric fibres. The layers of fabric with rigid warp and weft provide both the transmission of the shear movement to the members on which the actuator is installed and the rigidity of the device in the other directions.

The drawback of this device is the limited amplitude of the angular distortion allowed by an elementary module of this type of actuator. Whatever the piezoelectric material used in the fibres, the tests carried out by the applicant result in a mean angular distortion of 1256 μdef at 45° for a fibre deformation of 800 μdef.

The object of the invention is to obtain much greater values of the angular distortion with an actuator using a layer of piezoelectric fibres. This requires firstly increasing the value of the angular distortion produced by the movement of the piezoelectric fibres, secondly guaranteeing the level of force produced by the actuator in the shear direction.

To this end, the invention relates to a piezoelectric actuator with a large shear movement in a chosen direction, having a sandwich structure comprising at least one active layer comprising piezoelectric fibres parallel to one another and inclined with respect to said direction, said active layer being placed between at least two layers carrying electrodes disposed so as to be able to cause on command a variation in length of said fibres. This actuator is characterised in that:

-   -   the angle of inclination of said fibres with respect to said         chosen direction is greater than 2° and less than 40°;     -   the spaces between piezoelectric fibres in the active layer are         filled with an incompressible elastic material with a dielectric         strength greater than that of said fibres;     -   said active layer comprises at least two elements referred to as         non-deformable elements, these elements being elongate and         parallel to said chosen direction and having a tensile modulus         of at least 200 GPa;     -   the ends of each of said fibres are adhesively bonded to said         non-deformable elements by means of a rigid glue;     -   said non-deformable elements are adhesively bonded, with a rigid         glue, to said electrode-carrying layers.

The combination of non-deformable elements and the matrix consisting of the incompressible elastic material in the inter-fibre spaces makes it possible to ensure that the active layer reacts to the tensile forces exerted in its plane, substantially like a succession of deformable parallelograms with rigid edges between two non-deformable elements.

In this configuration, it can be shown that inclining the piezoelectric fibres in a direction of less than 40° with respect to the direction of the non-deformable elements makes it possible to obtain angular distortions greater than the diagonal inclination at 45° by a factor of at least 2.

Finally, the transmission of the forces to the members to which the actuator is applied, passing through the electrode-carrying layer, takes place entirely through the rigid elements, through the rigid glues. These elements must therefore have a high tensile modulus in order to withstand, without deforming, the forces induced by the fibres. On the other hand, this transmission mode eliminates the losses caused by the shear strength in the layers of glue responsible for transmitting the forces between the active layers and the other elements of the sandwich, as exist in the prior embodiments.

Advantageously, the angle of inclination of said piezoelectric fibres with respect to said chosen direction is less than 10°. This is because the angular distortion increases greatly when the inclination of the fibres decreases and is theoretically four times greater than the value obtained for 45°, starting from this inclination.

Preferably, the piezoelectric fibres are connected to the electrode-carrying layer by an elastic material having a shear modulus of less than 10 MPa. This layer of elastomer makes it possible to avoid electrical breakdowns by keeping the distance between the piezoelectric fibres and the electrodes substantially constant. On the other hand, its low modulus means that it opposes a low shear strength to the movement of the fibres and that it therefore reduces the force exerted by the actuator only a little.

Preferably, the electrode-carrying layer is produced from non-conductive material manufactured from polyamide. This type of material withstands well the angular shearing imposed by the actuator in operation.

Advantageously, the two edges of the actuator that are parallel to the chosen direction of said active layer are each formed by one of said non-deformable elements. This configuration has the advantage, inter alia, that the movements can be transmitted to the actuated members only by these two non-deformable elements.

The invention also relates to a method for producing a piezoelectric actuator with a large shear movement in a chosen direction, having a sandwich structure comprising at least one active layer comprising piezoelectric fibres parallel to one another and inclined with respect to said direction, said active layer being placed between at least two electrode-carrying layers disposed so as to be able to cause on command a variation in length of said fibres. This method is characterised by the following operations:

-   -   producing a said active layer, by:         -   positioning said piezoelectric fibres with an angle of             inclination greater than 2° and less than 40° with respect             to said chosen direction;         -   filling the spaces between piezoelectric fibres with an             incompressible elastic material with a dielectric strength             greater than that of said fibres;         -   placing, in parallel with said chosen direction, at least             two elements from said non-deformable elements, these             elements being elongate and having a tensile modulus of at             least 200 GPa;         -   adhesively bonding, with a rigid glue, the ends of each of             said fibres to said non-deformable elements;     -   adhesively bonding, with a rigid glue, said non-deformable         elements to said electrode-carrying layers.

Advantageously, the step of positioning the piezoelectric fibres is performed by sawing a sheet of the material making up said piezoelectric fibres manufactured substantially to the dimensions of the active layer.

Preferably, the non-deformable elements are placed in grooves formed in the shearing direction in a sheet substantially to the dimensions of the active layer composed, over its entire extent, of piezoelectric fibres separated by bands of elastic material.

According to these last two features, producing a block makes it possible to save on time while ensuring regular positioning of the elements and evenness of the actuator on its surface.

The invention also relates to an at least partially deformable aerodynamic element comprising a piezoelectric actuator as described above.

The present invention will be understood better and other details, features or advantages of the present invention will become more apparent from reading the following description with reference to the accompanying drawings, in which:

FIG. 1 schematically shows the structure in superimposed layers of a rectangular planar actuator according to the invention.

FIG. 2 schematically shows an exploded view of the various layers making up the actuator.

FIG. 3 shows a schematic detail of the active layer containing the piezoelectric fibres.

FIG. 4 shows a schematic detail of the intermediate layer providing the connection between the active layer and the electrode-carrying layer.

FIG. 5 schematically illustrates the theoretical deformation of an articulated rigid frame subjected to elongation of its diagonal.

FIG. 6 shows a schematic comparison between the angular distortions of a rigid frame (C1) and of a sandwich formed by two rigid bars connected by an elastomer (C2), obtained for various values of inclination of the diagonal.

FIG. 7 schematically shows the steps of manufacturing an actuator according to the invention.

An example of a planar actuator, shown in FIG. 1, has a rectangular form of thickness t in the direction Z, of length in the direction X, which represents its largest dimension, and of width w in the direction Y. This actuator has a sandwich structure composed of the following elements:

-   -   in the central plane XY, a layer 1 comprising the piezoelectric         fibres 2,     -   planar layers, 3 and 4, carrying the electrodes 5 activating the         piezoelectric fibres 2 placed on either side in the direction Z,         and     -   intermediate layers, 6 and 7, of glue providing the connection         between the central layer and the electrode-carrying layers.

The central layer 1 (FIG. 2) is composed of an alternation in the direction Y of bands comprising the piezoelectric fibres 2 and of filiform bars 8. Each long edge, in the direction X, of the central layer 1 is formed by a filiform bar. This layer is preferably approximately 180 μm thick.

The filiform bars 8 have a rectangular cross section, with a width of around 350 μm, and are parallel to the long edge in the direction X and spaced apart, for example, by 1 to 2 mm. These filiform bars 8 are made from highly insulating materials with a high tensile modulus, of at least 200 GPa, which makes them non-deformable elements. Materials with a high dielectric strength and a high tensile modulus such as silicon carbide and boron are preferably used.

Between these filiform bars 8, piezoelectric fibres 2 with a rectangular cross section are disposed, forming an angle θ with the direction X. This angle is chosen in the range between 2° and 40°. The properties of the actuator according to the value chosen are disclosed later, during the description of its functioning. For example, the width of these fibres is around 150μ. The material of these fibres 2 is of the MFC (macro-fibre composite) type, preferably to be taken from the following list: PZT-SA, PZT-5H or PMN-32% PT single crystal.

The piezoelectric fibres 2 are adhesively bonded at both ends thereof to the filiform bars 8 with epoxy rigid glue 9.

The gap between two fibres 2 is filled with an elastomer 10, which is preferably incompressible, with a low shear modulus, preferably less than 20 MPa. The dielectric strength of this elastomer 10 is also preferably greater than that of the piezoelectric fibres 2. The width of the band of elastomer 10 between two fibres 2 is preferably approximately 55μ. The minimum width of these gaps is limited firstly by the need for the elastomer to withstand the deformation of the actuator, and secondly by the dielectric strength of this elastomer 10, so that there is no electrical breakdown between the regions with opposite polarities on the piezoelectric fibres 2.

The planar layers 3, 4 carrying the electrodes 5 are formed by a thin film of polyamide material of the Kapton (registered trade mark) type. Preferably, the thickness of the film is approximately 0.3 mm. The electrodes are embedded in the film of the planar layer and are formed by two series of elongate interdigitated electrodes (51 and 52 for the top layer and 3, 53 and 54 for the bottom layer 4). These electrodes 5 are oriented perpendicular to the piezoelectric fibres 2.

The intermediate layers 6 and 7, providing the connection between the layer 1 of piezoelectric fibres and the electrode-carrying layers 3 and 4, are composed, as depicted in FIG. 3, of parallel bands in the direction X:

-   -   at the space occupied by the piezoelectric fibres 2, by an         elastomer 11 with a low shear modulus, typically less than 10         MPa;     -   at the filiform bars 8, by a rigid glue of the epoxy type.

The thickness of these layers, for example around 20 is limited to what is necessary to ensure the rigid connection between the filiform bars 8 and the electrode-carrying layers 4 and 5.

The functioning of the actuator according to the invention is such that, when the piezoelectric fibres 2 are energised by means of the interdigitated electrodes 51, 52, 53 and 54, secured to the electrode-carrying layers 4 and 5, made from Kapton, the fibres 2 elongate while bearing at each of their ends on the filiform bars 8, which move in opposite directions.

Unexpectedly, the assembly formed by the filiform bars 8 and the elastomer 10 placed between the piezoelectric fibres 2 behaves substantially as a deformable frame having rigid edges, the diagonal of which is inclined by an angle θ with respect to the long edges consisting of filiform bars 8. When such a frame, depicted in FIG. 5, is subjected to a longitudinal deformation ∈_(L) imposed in the direction of its diagonal, the angular distortion γ obtained increases very quickly when the inclination θ of this diagonal decreases. FIG. 6 shows a comparison of the value of the angular distortion γ obtained for a deformation ∈_(L) equal to 800 μdef, with a rigid frame by an analytical calculation (C₁), and with a frame consisting of two rigid elements connected by an elastomer similar to the one used between the piezoelectric fibres 2 by a numerical calculation (C₂). The very similar results of angular distortions show that the active layer 1 can behave substantially as a rigid frame articulated by means of the elastomer 10 situated between the fibres 2.

Moreover, these results illustrate the fact that, for a given elongation of the piezoelectric fibres 2, it is possible to obtain appreciably greater angular distortions when the inclination of the fibres is small. In addition, this shearing is transmitted to the electrode-carrying Kapton layers since they are adhesively bonded to the filiform bars 8 by the epoxy glue 12 with a high rigidity modulus.

Moreover, the lower the forces opposing the elongation of the fibres, the greater is such elongation. The shearing created by the actuator at its centre is therefore greater when:

-   -   the shear modulus of the elastomer 9 between the piezoelectric         fibres 2 is small, thus reducing the rigidity of the frame         formed with the filiform bars 8;     -   the shear modulus of the elastomer 11 connecting the fibres 2 to         the electrode-carrying layers 3 and 4 is small, thus reducing         the influence of the rigidity of the electrode-carrying layers         on the deformation of the piezoelectric fibres.

Because of the evaluation of the rigidity of the assembly opposing the work of the fibres 2 and because of the angular distortion value γ for small angles θ, an actuator according to the invention is preferably produced with an inclination of the fibres of less than 10°. However, the smaller this angle, the smaller the number of fibres 2 for a given actuator surface area. It therefore appears that there will be a value below which the force exerted by the fibres will not be sufficient to overcome the shearing of the elastomer 9 between the filiform bars 8. It is for this reason that a limit 2° less than this inclination is fixed here for a practical embodiment.

In order to illustrate the performance that can be expected of a sensor according to the invention, calculations were carried out to model the response of an elementary module formed by two filiform bars 8 connected by piezoelectric fibres inclined at 10°. The two modules differ on account of their lengths and widths: 5.2×1.2 mm for the first, 10.5×2.2 mm for the second. Otherwise, the other characteristics, summarised in the following table, are identical:

Thickness of the active layer 1 180μ  Thickness of the electrode-carrying layers 3 and 4 20μ Thickness of the connecting layers 6 and 7 30μ Shear modulus of the elastomer (polyol) 10 between the   0.1 MPa adjacent fibres, and 11 between the fibres and the electrode-carrying layers Shear modulus of the epoxy glue 9 between the filiform bars 1200 MPa and the fibres, and 12 between the filiform bars and the Kapton layers Tensile modulus of the material (steel) of the filiform bars 8  210 GPa

The other dimensional characteristics are those already cited as an example in the description of the embodiment. The piezoelectric fibres 2 are made from 1000 volt PZT5A so as to obtain a fibre elongation, denoted ∈_(L), of around 800 μdef. The electrode-carrying layers 3 and 4 are made from Kapton.

The angular distortion is evaluated at the centre of the Kapton film forming an electrode-carrying layer 3. This value is compared with that obtained with a sensor according to the prior art, the fibres of which are inclined at 45° and also subjected to a voltage of 1000 V. This sensor has a length of 85 mm and a width of 28 mm. The results are set out in the following table:

Angular Orientation Elongation distortion of the Type of actuator of fibres of fibres ε_(L) Kapton γ Gain Prior art 45° 800 μdef 1200 μdef 1 First module 10° ≈800 μdef 2360 μdef 2 Second module 10° ≈800 μdef 2450 μdef 2

Moreover, the method for manufacturing the example of an actuator described previously is illustrated in FIG. 7. It uses at the start a sheet 13 of PZT adhesively bonded to a polymer film 14, such as can be obtained by a known manufacturing method. This sheet is to the dimensions of the active layer 1 of the actuator finally produced.

A first step A consists of sawing this sheet in order to form parallel strips 2 of piezoelectric fibres, inclined by an angle θ with respect to the long edge of the sheet. The separation of the fibres 2 corresponds to their separation in the actuator. Moreover, complete sawing of the polymer film 14 supporting the assembly is avoided.

In a step B, the interstices between piezoelectric fibres 2 are filled with the elastomer 10 of the active layer 1.

In a step C, sawing is carried out in the direction of the length of the assembly, fibres 2 and elastomer 10, without attacking the polymer film 14, in order to form longitudinal grooves in as many locations as there will be filiform bars 8.

Step D consists of depositing a protective cover 15 on the longitudinal bands comprising the fibres 2.

Step E consists of placing the filiform bars 8 in the longitudinal locations and adhesively bonding them to the piezoelectric fibres 2 with the epoxy glue 9.

Step F consists of removing the protective covers 15.

Step G consists of placing the films of epoxy glue 12 on the top face of the active layer 1 thus created, opposite the filiform bars 8, and placing the elastic films 11 opposite the bands containing the piezoelectric fibres 2.

Step H consists of placing the electrode-carrying layer 3 with its electrodes 5 and then connecting it to the active layer 1 by a polymerisation of the films 12 and 11.

During a step I, the polymer film is removed and then in a step J the two layers thus obtained are turned over.

Next, steps G and H are performed on the other side of the active layer 1 in order to place the second electrode-carrying layer 4.

This type of actuator can advantageously be used in aeronautical applications, in particular for modifying the shape of an aerodynamic element. For example, the patent application FR 2 924 681 describes the use of planar actuators inserted in the structure of a blade of a rotating wing in order to deform it torsionally by a relative sliding of the parts situated around a slot oriented substantially in the direction of the wing span. In addition, the amplitude of the sliding must be controlled during the rotation of the blade. The actuator according to the invention can easily be substituted for the planar actuator of the piezoelectric type already envisaged in this application. In general terms, the invention can be applied whenever a part of an aerodynamic element must perform sliding or rotation movements about a given position and the space allocated to the actuator is small. 

1. Piezoelectric actuator with a large shear movement in a chosen direction, having a sandwich structure comprising: at least one active layer comprising piezoelectric fibres parallel to one another and inclined with respect to said direction, said active layer being placed between at least two layers carrying electrodes disposed so as to be able to cause on command a variation in length of said fibres, wherein the angle of inclination of said fibres with respect to said chosen direction is greater than 2° and less than 40°; the spaces between piezoelectric fibres in the active layer are filled with an incompressible elastic material with a dielectric strength greater than that of said fibres; said active layer comprises at least two elements, the at least two elements being elongate and parallel to said chosen direction and having a tensile modulus of at least 200 GPa; the ends of each of said fibres are adhesively bonded to said at least two elements by a rigid glue; said at least two elements are adhesively bonded, with a rigid glue, to said layers carrying electrodes.
 2. Actuator according to claim 1, wherein the angle of inclination of said piezoelectric fibres with respect to said chosen direction is less than 10°.
 3. Actuator according to claim 1, wherein the piezoelectric fibres are connected to the electrode-carrying layer by an elastic material having a shear modulus of less than 10 MPa.
 4. Actuator according to claim 1, wherein the electrode-carrying layer is produced from a non-conductive material that includes a polyamide.
 5. Actuator according to claim 1, wherein the two edges parallel to the chosen direction of said active layer are each formed by one of said at least two elements.
 6. Method for producing a piezoelectric actuator with a large shear movement in a chosen direction, having a sandwich structure comprising at least one active layer comprising piezoelectric fibres parallel to one another and inclined with respect to said direction, said active layer being placed between at least two layers carrying electrodes disposed so as to be able to cause on command a variation in length of said fibres, said method comprising: producing one said active layer, by: positioning said piezoelectric fibres with an angle of inclination greater than 2° and less than 40° with respect to said chosen direction; filling the spaces between piezoelectric fibres with an incompressible elastic material with a dielectric strength greater than that of said fibres; placing, in parallel with said chosen direction, at least two elements, the at least two elements being elongate and having a tensile modulus of at least 200 GPa; adhesively bonding, with a rigid glue, the ends of each of said fibres to said at least two elements; adhesively bonding, with a rigid glue, said non-deformable elements to said layers carrying electrodes.
 7. Method according to claim 6, wherein said positioning the piezoelectric fibres is performed by sawing a sheet of the material making up said piezoelectric fibres manufactured substantially to the dimensions of the active layer.
 8. Method according to claim 6, wherein the at least two elements are placed in grooves formed in the shearing direction in a sheet substantially to the dimensions of the active layer composed, over its entire extent, of the piezoelectric fibres separated by bands of elastic material.
 9. At least partially deformable aerodynamic element comprising a piezoelectric actuator according to claim
 1. 10. Method according to claim 6, wherein the at least two elements are non-deformable elements.
 11. Method according to claim 6, wherein the at least two elements are dimensionally stable.
 12. Actuator according to claim 1, wherein the at least two elements are non-deformable elements.
 13. Actuator according to claim 1, wherein the at least two elements are dimensionally stable. 